Contemporary paging systems employ messaging schemes that can deliver voice, numeric, or alphanumeric messages to a user. The majority of paging systems transmit address and message information using a protocol such as GSC (Motorola's Golay Sequential Code) or POCSAG (Great Britain's Post Office Code Standardization Advisory Group). To originate a message or page, the base station or paging terminal is typically accessed via the PSTN from a telephone. As a voice message entry device, the telephone is acceptable but when data needs to be entered, an alternative means of entry is desirable. Alternative entry devices such as computer terminals or custom entry devices work well if the originator can convey information to the user in a textual format. Unfortunately, when a user must convey a large amount of information, existing paging systems and data transport protocols do not allow for the transmission of either long textual messages or messages containing graphical data. Thus, for reasons associated with the data entry problem, most paging service providers do not provide alphanumeric paging message services. To display the message, the selective call receivers need high information content displays that require a large number of picture elements (pixels) to portray arbitrary patterns of information. Recent technological advances have produced LCDs with time constants approaching the frame period used in many video displays (approximately 16.7 milliseconds). Such a short time constant allows the LCD to respond quickly and is especially advantageous for depicting motion without noticeable smearing of the displayed image.
When the display is an active addressing display, it can be shown mathematically that there is applied to each pixel of the display a root-mean-square (rms) voltage averaged over the frame period, and that the rms voltage is proportional to each pixel value for the frame. The advantage of active addressing is that it restores high contrast to the displayed image because instead of applying a single, high level selection pulse to each pixel during the frame period, active addressing applies a plurality of much lower level (2-5 times the rms voltage) selection pulses spread throughout the frame period. In addition, the much lower level of the selection pulses, the probability of alignment instabilities are substantially reduced.
A problem, however, with active addressing results from the large number of calculations required per second. For example, a gray scale display having 480 rows and 640 columns, and a frame rate of 60 frames per second requires just under ten billion calculations per second. While it is of course possible with today's technology to perform calculations at that rate, the architecture proposed to date for calculation engines used for active addressed displays have not been optimized to minimize power consumption. The power consumption and calculation speeds are issues of particularly importance to display units of portable devices that are powered by limited energy battery power supplies. To compound this problem, when facsimile communication is being received by the selective call receiver, the selective call receiver which is a portable battery powered device must also receive, decompress, and decode the data while performing a large number of calculations to display the data on the electronic display.
Thus, what is needed is a method and apparatus, in a portable device, for facsimile communication that does not increase the power consumption for displaying large amount of data.